The present invention relates generally to an output power meter for a nuclear reactor, and more particularly to an output power level measuring device for use with a light water reactor (LWR), which is specifically designed to measure the output power of a nuclear reactor by the detection of an immediate concentration of a radioisotope .sup.16 N in the primary coolant of the reactor.
There is already known as disclosed in, for example, the Official Gazette of Japanese Patent Laid-open Application No. 47-13,600 corresponding to the U.S. patent application Ser. No. 102,617 filed on Dec. 30, 1970, a system for measuring the output of a nuclear reactor by way of the detection of the concentration of a radioisotope .sup.16 N contained in the primary coolant of the reactor.
While this system of measuring a reactor output is generally quicker in response in comparison with the common thermal output measuring sytem, from which there is advantageously attainable a current measurement in direct proportion to the reactor core output, it is known that this method is not convenient, since an output response to a variation in a transient output of a nuclear reactor as shown in FIG. 1B resuls in a stepwise cumulation as shown by a curve in FIG. 1C. This is primarily because the primary cooling water to be measured is subjected in repetition to irradiation of the neutron, while circulating in the closed loop of a cooling system. As a consequence, in order to obtain a proper output response which is proportional to the current variation in the reactor output as a function of time, it is generally required to provide a compensatory operation for such a cumulation in the measuring response by using either an analog type electronic computing circuit or an electronic digital computer. Preferably, this compensative operation is to be conducted by way of the former analog type electronic computing circuit rather than by the latter electronic digital computer from the viewpoints of reliability and quickness in responses. As it is the practice that the reactor output meters are generally incorporated in a safeguard system of a reactor, it is highly desirable to employ the analog type electronic computing circuit rather than the use of the electronic digital computer, accordingly.
However, in consideration of the fact that the cumulative phenomenon in transient responses of the reactor output measuring system as reviewed hereinbefore in connection with FIG. 1 is from an intrinsic nature of process which reflects a pure delay time, it would then be difficult to compensate for the cumulative phenomenon without the use of an electronic digital computer. This comes true with the use of the electronic computing circuit of analog type in the prior art noted above, namely the accuracy of compensation for such a cumulative phenomenon turns out to be deficient, after all.
Next, FIG. 2 shows, by way of one example of the prior art, a result of compensation, in which curve 1 represents a state of responses in measurement of a reactor output prior to a compensatory operation, and curve II represents a state of responses after a due compensation therefor. As appreciated from the curve II, there is seen left still an error of 10% or more in the response to the reactor output measurement, which means an obvious deficiency in accuracy as for a reactor output meter, accordingly.
Now, this cumulative phenomenon in transient responses to a current measurement of variations in the reactor output will be reviewed in further detail in conjunction with FIG. 1. In FIG. 1A, there are shown a nuclear reactor designated by reference numeral 1, a steam generator by 2, a gamma (.gamma.) ray detector by 3, a reactor core by 4, and a primary coolant loop by 5 in which a primary cooling water circulates.
There is generated radioisotope .sup.16 N when the primary cooling water is subjected to the irradiation of neutrons while circulating in the area of the core 4 of the nuclear reactor 1, which radiates the gamma rays. Thus generated .sup.16 N is then carried along with the circulation of the cooling water, and then is measured for its concentration by the gamma ray detector 3 mounted on a delivery piping (not shown) at the outlet of the reactor core 4. In general, it is known that a typical time of circulation of the primary cooling water through the primary coolant loop 5 is approxiately 10 sec. in the case of a typical pressurized water reactor (hereinafter, referred to as "PWR"). Also it is known that the half life of .sup.16 N is approximately 7 sec. Therefore, at the moment that the primary cooling water returns back to the reactor core 4 after it is once subjected to the irradiation of the neutrons in the reactor core 4 and after the circulation throughout the cooling loop, the quantity of .sup.16 N generated last will be left by approximately 35% thereof. With repetition of this process of circulation, a change in the concentration of .sup.16 N with respect to a stepwise variation in the reactor power level as shown in FIG. 1B would, as reviewed generally hereinbefore, exhibit cyclically a stepwise cumulation converging towards a stationary value as typically shown in FIG. 1C. The wve form as observed at each of the rise portions of this stepwise ascending curve has a spread which is attributed to a period of time that the primary cooling water passes through the reactor core and a spread which is attributed to the effect of agitation of the cooling water caused during a number of circulations through the cooling loop. Further to this aspect, reference will then be made to FIG. 3.
When there occurs a momentary change or increase in the output of a nuclear reactor (at time t=0 as shown in FIGS. 3B and 3D, there is observed a change in the concentration of .sup.16 N at this moment between both upstream and downstream portions of the cooling water currently existing in the area of reactor core 4. When this boundary of change in concentration of .sup.16 N is passed in front of the gamma ray detector 3 by the flow of cooling water, the output signal of this gamma ray detector 3 may change or increase stepwise (as, for instance, shown at time T.sub.1 shown in FIGS. 3D and 3E, respectively). It is to be noted that this boundary is initially spread to a certain extent of time according to the size of the reactor core 4, and as the cooling water flows along the primary coolant loop 5, this extent of spread would gradually further grow owing to the effect of the turbulence around the current boundary as encountered during the circulation through the coolant loop. This spread of boundary may appear as a rise time in the output signal of the gamma ray detector 3 as shown in FIG. 3E. When this boundary passes in front of the gamma ray detector 3 and arrives again this gamma ray detector 3 after having circulated once the entire primary coolant loop 5 at the time of (t=T.sub.1 +T.sub.2), it is observable that the current rise time would turn to be longer than the last one under the effect of agitation of the cooling water recirculating through the coolant piping, and also that the increment of the rise may be reduced owing to the attenuation of .sup.16 N according to its half-life. Among these two aspects of spread of boundary (the rise time in responses) owing to two different causes as noted above, the one owing to the size of the reactor core 4 would depend upon the distribution of the output of the core 4 of the nuclear reactor 1, and the spread configuration would exhibit nearly the normal distribution. The other owing to the effect of agitation rendered by the cooling water during the passing thereof through the piping is of the phenomenon of diffusion given by the turbulent flow, and consequently, the spread configuration thereof may turn out to be the normal distribution.
Also, it is observed that the configuration of each rising portion of the stepwise curve shown in FIG. 3E is obtained from the condition of the above various factors, and that that of the one owing to the effect of agitation of the cooling water is obtained from the convolution corresponding to the number of the circulation in the coolant loop. Besides, as the input curve is of a step waveform, the configuration of each rise portion in FIG. 3E turns out to be nearly of an integral of normal distribution function (an error function).
Now, assuming that a spread of boundary owing to the passing through the reactor core 4 be SFc, a spread of boundary owing to the effect of agitation on the cooling water as observed after the reactor core 4 up to the point of the gamma ray detector 3 be SF.sub.1, and a spread of boundary owing to the effect of agitation on the cooling water observed in the circulation through the entire coolant loop 5 be SF.sub.2, the response as shown in FIG. 1C may be represented by the following equation. Here, the convolution is simplified by a symbol " ". Also, equations may be represented with this symbolization in the following manner: that is, ##EQU1## where, U(t): a unit step function
.delta.(t): a unit delta function PA1 T.sub.1 : a time spent by the flow of the cooling water from the reactor core 4 to the gamma ray detector 3 PA1 T.sub.2 : a time required for the cooling water to circulate the entire primary coolant loop 5 PA1 .lambda.: an attenuation coefficient of .sup.16 N (approx. 0.1 sec.sup.-1)
From the equation (1), a correction for the purpose of removing the stepwise cumulative aspect of responses to measurement may be given by the following equation. That is; EQU e.sup..lambda.T.sbsp.1 S(t) {.delta.(t)-e.sup.-.lambda.T.sbsp.2 .delta.(t-T.sub.2) SF.sub.2 }=U(t-T.sub.1) SFc SF.sub.1 ( 2)
More specifically, substituting the equation (1) for the left member of the equation (2), there is obtained a response output with no term exhibiting any stepwise aspect in the right member of the equation (2). It should be noted that this correction is effective only for the removal of the questioned stepwise cumulation, but the terms SF.sub.1 and SFc among the terms SF.sub.1, SF.sub.2 and SFc are left as they are. The present invention provides an improved nuclear reactor output power meter in accordance with this correction. In order to carry out this corrective arithmetic operation, it will be required to similate a pure delay time in this operation, so that it will be impractical to perform this operation by way of the aforementioned analog type electronic arithmetic circuit. Moreover, the configuration of the spread SF.sub.2 is the left member of the equation (2) is not of a first order lag which can readily be realized by way of the analog electronic arithmetic circuit but of nearly the normal distribution function, and consequently, it is difficult to attain such a simulation by using the analog type electronic arithemtic circuit.
According to the conventional arrangement as noted above in connection with FIG. 2 for this correction system using such an analog type electronic arithmetic circuit, it employs a multi-cascade connection of first order lag elements as a delay element. However, since it remains unclear how to determine the number of the cascade connection, a certain number of the cascade connection has been set without any basis thereof, which may turn out to be a rather poor accuracy as reviewed hereinbefore in connection with the correction curve II of FIG. 2. More specifically, the curve I of FIG. 2 shows a step response of the .sup.16 N detector 3 in a typical PWR plant while the curve II shows the result of the correction for the above response by means of such a correction as noted above. With this example, even when the number of the cascade connection of the first order lag element is somewhat increased, the accuracy of correction is insufficiently improved, which has been thought to be the inherent limit in the attainable response for this type of system.
In this respect, therefore, there have been two essential requirements in the conventional art for the correction of response in the measurement, that is the pure delay time and the normal distribution response, and since there are required an infinite number of circuit elements for meeting these two requirements by way of an analog type circuit, it has been difficult in practice to realize this concept.